These sub-contractors then hire workers on the condition they take out their own Australian Business Number (ABN) — effectively making them sub-contractors as well.

Each layer takes a cut of the original contract sum, until the workers at the bottom find themselves getting paid well under the minimum rate.

The union is calling on the Government to intervene.

"The Andrews Government is responsible for what is happening in our schools — it is Victorian government schools," Ms Walsh said.

"They're presiding over a system that is causing widespread wage theft and widespread harm to the cleaning workforce."

The union is urging the Government to follow the New South Wales model, where the entire public school system is covered by 22 contracts, making regulation and compliance a much simpler process.

Victoria's Education Minister James Merlino said the Government was working with the union to address this issue.

"It is always concerning to hear reports that some cleaners may have been underpaid or intimidated," he said.

"This is not acceptable and I would strongly urge cleaners who are concerned about being underpaid or intimidated in any way to immediately contact the Department of Education's cleaning contract unit to investigate."